By fixed echoes are meant the signals received by a radar which originate from fixed or slow moving targets such as buildings, hills, trees, as well as clouds, angel echoes or waves, all these being unwanted signals which are referred to as "clutter". This term will be used in the following description to designate such unwanted signals which, when mixed with useful signals, tend to mask them. Thus, originating as they do from fixed or very slow-moving targets, these clutter signals must be suppressed if radar equipment is to be operated properly.
A number of arrangements for suppressing fixed echoes in a radar are known and it will be helpful to refer to the analyses and descriptions of them which appear for example in volume 2, chapter 13 of "Physique et theorie du radar" by J. Darricau, or in chapter 17 of the "Radar Handbook" by Merril I. Skolnik.
However, it generally appears that the dynamic range of signals which can effectively be dealt with by means of fixed-echo cancelers is very much less than the dynamic range of signals reflected by fixed clutter. It will be recalled that by the dynamic range of signals is meant the maximum variation in their amplitude.
Thus, to enable fixed echoes to be canceled satisfactorily, it is necessary to reduce the dynamics of the signals to be handled. To solve this particular problem various methods have been suggested and used; mention may be made, inter alia, of amplitude-limiting means in an intermediate-frequency radar amplifier having a lin-log (or linear-logarithmic) characteristic, amplifiers whose gain is variable with time (GTC amplifiers) and which are programmed as a function of the level of fixed echoes at a given radar site, and automatic-gain-control (AGC) circuits.
However, gain-limiting circuits in the intermediate-frequency amplifier of the radar have the drawback of severely reducing the performance of fixed-echo suppressors because they operate under non-linear conditions. Amplifiers whose gain is variable with time and which are programmed in a set fashion have the disadvantage of being ill-adapted to changes in the levels of fixed echoes due to fluctuations in the propagation of the radio-electrical waves.
Automatic-gain-control circuits are also well known in radio receivers, in which the bipolar video signal obtained from the output of the detector connected to the linear intermediate frequency amplifier is rectified and integrated over a few radar pulses and the resulting signal is applied as negative feedback to the intermediate-frequency amplifier so as to control its gain, have the drawbacks of being difficult to implement and of relatively poor performance.